Preparation of N-acylamino carboxylic acids and N-acylamino sulfonic acids and their alkali metal salts

ABSTRACT

N-acylamino carboxylic acids and N-acylamino sulfonic acids and their alkali metal salts from the technical alkali metal salts of amino carboxylic acids and amino sulfonic acids, respectively, with an active content of 50-95% by weight, based on the solids content of the technical alkali metal salts, and from alkyl carboxylates, are prepared by 
     (a) preparing a suspension of the solid anhydrous technical alkali metal salts of the amino carboxylic acids or amino sulfonic acids in the alkyl carboxylates, 
     (b) reacting this suspension by adding more than 30 to 150 mol % of strong bases to give the alkali metal salts of the N-acylamino carboxylic acids or N-acylamino sulfonic acids, and 
     (c) if required preparing therefrom the free N-acylamino carboxylic acids or N-acylamino sulfonic acids in a conventional way by adding acids.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved process for preparingN-acylamino carboxylic acids and N-acylamino sulfonic acids and theiralkali metal salts from the technical alkali metal salts of aminocarboxylic acids and amino sulfonic acids, respectively, and from alkylcarboxylates.

2. Discussion of the Background

DE-A 20 04 099 discloses that salts of N-acylamino carboxylic acids canbe prepared by acylation of the corresponding amino carboxylic acids ortheir salts with a carboxylic acid, a carboxylic ester or a carboxamidein the presence of an equivalent amount of a basic compound which formssalts with carboxylic acids, such as an alkali metal or alkaline earthmetal hydroxide, at from 100° to 250° C., preferably from 160° to 200°C. In this case the basic compound merely serves to neutralize the aminocarboxylic acids or carboxylic acids used. The reaction is carried outin the melt of amino carboxylic acid and acylating agent or in asuspension of the amino carboxylic acid in a solution of an acylatingagent and of a basic organic nitrogen compound, eg. an amine, in ahigh-boiling organic solvent.

However, the disadvantages of the described reaction procedure are thelong reaction times and the low yields, which are attributableessentially to the partial decomposition of the starting compounds andof the products because of the high reaction temperatures. This isbecause the reaction mixture becomes slightly dark in color during thisand a slight evolution of carbon dioxide which is almost always observedcan likewise be explained only by decomposition reactions.

German Patent Application P 44 08 957.0 describes a process forpreparing N-acylamino carboxylic and sulfonic acids from the alkalimetal salts of the underlying amino carboxylic and sulfonic acids,respectively, and alkyl carboxylates, in which 0.5 to 30 mol % of strongbases are added as catalyst to a suspension of the amino carboxylic andsulfonic acid salts, respectively, in the carboxylates. However, thisprocess gives satisfactory results only on use of pure alkali metalamino carboxylates or sulfonates with an active content of more than 95%by weight, based on the solids content of the alkali metal salts. Use ofcorresponding salts of technical quality results in drastic losses ofselectivity of up to 15%.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an efficient andeconomic preparation process which starts from technical alkali metalamino carboxylates or sulfonates and which gives good space-time yieldsof N-acylamino carboxylic acids and N-acylamino sulfonic acids or theiralkali metal salts in high purities and selectivities.

We have found that this object is achieved by a process for preparingN-acylamino carboxylic acids and N-acylamino sulfonic acids and theiralkali metal salts from the technical alkali metal salts of aminocarboxylic acids and amino sulfonic acids, respectively, with an activecontent of 50-95% by weight, preferably 70-94% by weight, in particular80-93% by weight, based on the solids content of the technical alkalimetal salts, and from alkyl carboxylates, which comprises

(a) preparing a suspension of the solid anhydrous technical alkali metalsalts of the amino carboxylic acids or amino sulfonic acids in the alkylcarboxylates,

(b) reacting this suspension by adding 30 to 150 mol % of strong basesover the amount required for neutralization to give the alkali metalsalts of the N-acylamino carboxylic acids or N-acylamino sulfonic acids,and

(c) if required preparing therefrom the free N-acylamino carboxylicacids or N-acylamino sulfonic acids in a conventional way by addingacids.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The procedure in step (a) is, for example, for the alkyl carboxylatesand the solid anhydrous technical alkali metal salts of the aminocarboxylic acids or amino sulfonic acids to be introduced into asuitable vessel of glass or another material and processed with aconventional suspending agitator to give a fine-particle suspension.

However, the suspension can also be prepared in step (a) by mixingtogether the alkyl carboxylates and an aqueous solution of the technicalalkali metal salts of the amino carboxylic acids or amino sulfonic acidsin a suitable vessel of glass or another material, and subsequentlyremoving the water from the mixture as quickly as possible by heatingand applying a vacuum. It is possible to observe, surprisingly, thatduring this there is only very slight (usually less than 2%) hydrolysisof the alkyl carboxylates to the alkali metal salts of the underlyingcarboxylic acids. The advantage of using such aqueous solutions is thatit is possible to use aqueous solutions, produced in the industrialsynthetic process, of impure alkali metal salts of amino carboxylicacids or amino sulfonic acids without further processing such as spraydrying. In addition, the problems associated with the handling of solidsdo not arise, such as dust formation or uniform metering in.

Technical alkali metal salts of amino carboxylic acids contain a rangeof subsidiary components; in the case of, for example, technicalsarcosine sodium, which is usually prepared by the Strekker reaction ofmethylamine with formaldehyde and hydrocyanic acid and subsequenthydrolysis of the methylaminoacetonitrile formed, these are mainly otheramino carboxylates, eg. methyliminodiacetate or dimethylglycinate,carboxylates such as alkali metal salts of glycolic acid, and formate.

As a rule, the suspension in step (a) is prepared from equimolar orapproximately equimolar amounts of technical alkali metal salts of aminocarboxylic acids or amino sulfonic acids and alkyl carboxylates, anexcess of up to 15 mol %, in particular up to 10 mol %, of one of thetwo components being defensible. A larger excess of alkyl carboxylate,for example as diluent, is normally unnecessary.

The strong bases are added in step (b) to the suspension prepared instep (a) in order to start the reaction. The bases are usually addedafter or during the heating of the suspension to the reactiontemperature, but can also be added during the preparation of thesuspension in step (a), for example together with the alkali metal saltsof the amino carboxylic acids or amino sulfonic acids, or shortly beforethe heating of the suspension. The bases can be used as solid substancesor in dissolved form, for example in an organic solvent such as analcohol.

The amount of strong bases used is 30 to 150 mol %, preferably 70 to 135mol %, in particular 80 to 125 mol %, especially 90 to 115 mol %, overthe amount required for neutralization, based on the technical alkalimetal salts of the amino carboxylic acids or amino sulfonic acids. It ispossible to use a single species of a strong base or a mixture ofvarious bases.

Particularly suitable strong bases are:

alcoholates, especially alkali metal alcoholates of C₁ -C₄ -alkanols,eg. sodium methanolate, sodium ethanolate, sodium isopropoxide orpotassium tert-butoxide;

hydrides, eg. sodium hydride, sodium borohydride or lithium aluminumhydride;

alkali metal or alkaline earth metal hydroxides, eg. sodium hydroxide,potassium hydroxide, lithium hydroxide or calcium hydroxide;

alkali metal carbonates, eg. sodium carbonate, potassium carbonate orlithium carbonate;

amide salts, eg. lithium diisopropylamide;

organolithium compounds such as alkyllithium compounds, eg.n-butyllithium or methyllithium, or phenyllithium.

Of these, alcoholates are preferred.

The reaction in step (b) is carried out in a relatively mild temperaturerange, usually at 50° to 150° C., in particular at 80° to 140° C.,especially at 100° to 130° C. The reaction is normally carried out underatmospheric pressure; although autogenous pressure or elevated pressureis possible, it has no further advantages.

Alkyl carboxylate is usually no longer detected by analytical methods,eg. infrared (IR) spectroscopy, in the reaction mixture 1 to 2 hoursafter addition of the base. The alkanols produced in the reaction fromthe alkyl carboxylates usually distil out of the reaction mixture, whereappropriate under reduced pressure, or remain, at least in part, in thereaction mixture. After the reaction is complete, the reaction mixtureis normally in the form of a viscous paste. This can be dissolved afterthe temperature has been lowered, for example to 80° to 100° C., byadding water. This results, for example, in approximately 30-40% byweight aqueous solutions of the alkali metal salts of the N-acylaminocarboxylic acids or N-acylamino sulfonic acids.

If the intention is to obtain the free N-acylamino carboxylic acids orN-acylamino sulfonic acids, these are prepared in step (c) from thealkali metal salts in a conventional way by adding acids. Particularlysuitable acids are mineral acids such as sulfuric acid, phosphoric acidor hydrochloric acid, which are usually added at room temperature to theaqueous solutions of the alkali metal salts of N-acylamino carboxylicacids or N-acylamino sulfonic acids, so that the pH is adjusted to therange from about 0 to 3, in particular 1 to 2. This normally results inmilky creamy emulsions. These emulsions are advantageously separatedusing a conventional phase separation aid, eg. ketones such as isobutylmethyl ketone or methyl ethyl ketone, alkanols such as n-butanol,isobutanol or sec-butanol, ethers such as methyl tert-butyl ether ordiisopropyl ether, C₁ -C₄ -alkyl acetates, C₁ -C₄ -alkyl propionates oracetoacetic esters, which can be added at the same time as the acids orafter formation of the emulsion, at slightly elevated temperature, forexample at 40° to 70° C. Phase separation aids of this type are normallylow-boiling compounds which have little or no miscibility with water andcan be used at reasonable cost on the industrial scale.

The process according to the invention can be applied particularlysatisfactorily when the technical alkali metal salts of amino carboxylicacids used are the sodium or potassium salts of aliphatic aminocarboxylic acids having 2 to 10 carbons, preferably 3 to 6 carbons, inparticular of valine, leucine, norleucine, glycine, alanine, β-alanine,ε-aminocaproic acid, α-aminoisobutyric acid, sarcosine(N-methylglycine), aspartic acid, glutamic acid or iminodiacetic acid.However, it is also possible to use the sodium or potassium salts ofother natural α-amino acids, of oligopeptides or of aromatic orcycloaliphatic amino carboxylic acids, eg. anthranilic acid,phenylglycine, phenylalanine or 1-aminocyclohexane-1-carboxylic acid.Amino carboxylic acids in this connection are particularly compoundswith a primary or secondary amino group and one or two carboxyl groupsper molecule; however, it is also possible in principle to use compoundswith more than one amino group and/or more than two carboxyl groups, inwhich case the amount of alkyl carboxylates depends on the number ofamino groups. All the carboxyl groups are present virtually completelyin the salt form.

The process according to the invention can likewise be appliedparticularly satisfactorily when the technical alkali metal salts ofamino sulfonic acids are the sodium or potassium salts of aliphaticamino sulfonic acids having 2 to 10 carbons, preferably 2 to 4 carbons.of particular interest in this connection are the corresponding salts oftaurine (2-aminoethanesulfonic acid) and N-methyltaurine. Just like theamino carboxylic acids, it is possible for the amino sulfonic acidsused, which are likewise present virtually completely in the alkalimetal salt form, to have a plurality of amino groups and/or sulfonicacid groups.

Besides the sodium or potassium salts, it is likewise possible to usethe corresponding lithium salts.

Particularly suitable alkyl carboxylates are lower alkyl esters of fattyacids, ie. C₁ -C₄ -alkyl esters of saturated or unsaturated C₆ -C₃₀-monocarboxylic acids. Particularly suitable are the methyl esters ofsaturated or unsaturated C₈ -C₂₀ -monocarboxylic acids, eg. methyllaurate, methyl myristate, methyl palmitate, methyl stearate, methyloleate, methyl linoleate or methyl linolenate or mixtures of such estersof naturally occurring long chain fatty acids, eg. coconut fatty acidmethyl ester, palm oil fatty acid methyl ester, palm kernel oil fattyacid methyl ester, tallow fatty acid methyl ester, soybean oil fattyacid methyl ester, sunflower oil fatty acid methyl ester or rape seedoil fatty acid methyl ester, and corresponding esters of synthetic C₈-C₂₀ -monocarboxylic acids or mixtures thereof. The process according tothe invention can be carried out both in a batchwise and in a continuousprocedure.

The process according to the invention can be used to prepare pureN-acylamino carboxylic acids and N-acylamino sulfonic acids and theiralkali metal salts or mixtures of such compounds with different acylradicals and/or different basic amino carboxylic acids or amino sulfonicacids. Compounds of this type are known to be suitable as emulsifiers orsurfactants in a wide variety of industrial sectors.

The process according to the invention provides good space-time yieldsof the desired products of high purity. The relatively mild temperatureconditions mean that no decomposition reactions leading to reductions ofyield and darkening of color of the products occur. The reactions havegone to completion in a considerably shorter time.

The use of alkyl carboxylates as acylating components of reasonable costand ease of handling, and the avoidance of salt formation in theacylation make the process according to the invention, especially in acontinuous procedure, extremely attractive for the industrial scale.

A noteworthy point about the present process according to the inventionis that by increasing the content of strong bases in the reactionmixture beyond the normal catalytic amounts, the loss of selectivity ofthe reaction on use of technical, ie. impure, alkali metal aminocarboxylates or sulfonates in place of the pure salts is substantiallycompensated. This is all the more surprising since there is a prejudicein the literature that aliphatic carboxylic esters do not react withsecondary amines in the presence of equimolar amounts of base such assodium methanolate to give the corresponding amides (R. J. de Feoand andP. D. Strickler, J. Org. Chem. 28 (1963) 2915-2917). Example 2hereinafter, with sarcosine sodium, a secondary amine, clearlycontradicts this prejudice.

EXAMPLES Example 1 (for comparison)

Preparation of N-oleoylsarcosine using conventional catalytic amounts ofsodium methanolate

1 mole of methyl oleate (Edenor® ME TiO5 from Henkel) and 1 mole ofanhydrous technical sarcosine sodium powder (active content: 84.4% byweight sarcosine sodium) were placed in a beaker and mixed using anUltra-Turrax to give a fine suspension. The suspension was transferredinto a glass reactor and heated to 120° C. Under a nitrogen atmosphere,25 mol % of sodium methanolate as an approximately 30% by weightsolution in methanol were metered in with stirring over the course of 10minutes. After the addition was complete, the mixture was stirred at120° C. until carboxylate was no longer detectable in the IR spectrum(about 3.5 hours). The reaction mixture was then cooled to 100° C. and600 ml of water were added. The mixture was stirred until a clearaqueous solution was produced.

100 g of 96% by weight sulfuric acid were added to this solution. Acreamy emulsion formed, and this separated into a two-phase mixtureafter addition of 80 g of methyl ethyl ketone. This mixture wasseparated in a separating funnel, and the aqueous phase was discarded.The methyl ethyl ketone was distilled out of the organic phase at <50°C. and about 40 mbar and was reused. The residue was an orange-brown oilwith a content, determined by HPLC, of 76.7% by weight N-acylsarcosine;the yield was 290 g (corresponding to 83.5% of theory).

Example 2 (according to the invention)

Preparation of N-oleoylsarcosine using equimolar amounts of sodiummethanolate

The experiment described in Example 1 was repeated with the differencethat 100 mol % were used in place of 25 mol % sodium methanolate.Working up resulted in an orange-brown oil with a content, determined byHPLC, of 82.9% by weight N-acylsarcosine; the yield was 322 g(corresponding to 92.5% of theory).

Example 3 (according to the invention)

Preparation of N-oleoylsarcosine using above-stoichiometric amounts ofsodium methanolate

The experiment described in Example 1 was repeated with the differencethat 110 mol % were used in place of 25 mol % sodium methanolate.Working up resulted in an orange-brown oil with a content, determined byHPLC, of 85.2% by weight N-acylsarcosine; the yield was 320 g(corresponding to 91.9% of theory).

We claim:
 1. A process for preparing N-acylamino carboxylic acids andN-acylamino sulfonic acids and their alkali metal salts from thetechnical alkali metal salts of amino carboxylic acids and aminosulfonic acids, respectively, with an active content of 50-95% byweight, based on the solids content of the technical alkali metal salts,and from alkyl carboxylates, which comprises(a) preparing a suspensionof the solid anhydrous technical alkali metal salts of the aminocarboxylic acids or amino sulfonic acids in the alkyl carboxylates, (b)reacting this suspension by adding 30 to 150 mol % of strong bases overthe amount required for neutralization to give the alkali metal salts ofthe N-acylamino carboxylic acids or N-acylamino sulfonic acids, and (c)if required preparing therefrom the free N-acylamino carboxylic acids orN-acylamino sulfonic acids in a conventional way by adding acids.
 2. Aprocess as claimed in claim 1, wherein in step (a) a suspension ofequimolar or approximately equimolar amounts of technical alkali metalsalts of amino carboxylic acids or amino sulfonic acids and alkylcarboxylates is prepared.
 3. A process as claimed in claim 1, wherein instep (b) the strong bases are used in amounts of from 70 to 120 mol %,based on the technical alkali metal salts of the amino carboxylic acidsor amino sulfonic acids.
 4. A process as claimed in claim 1, wherein instep (b) alkali metal alcoholates are used as strong bases.
 5. A processas claimed in claim 1, wherein the reaction in step (b) is carried outat from 50° to 150° C.
 6. A process as claimed in claim 1, wherein thesodium or potassium salts of aliphatic amino carboxylic acids having 2to 10 carbons are used as technical alkali metal salts of aminocarboxylic acids.
 7. A process as claimed in claim 1, wherein the sodiumor potassium salts of aliphatic amino sulfonic acids having 2 to 10carbons are used as technical alkali metal salts of amino sulfonicacids.
 8. A process as claimed in claim 1, wherein the C₁ -C₄ -alkylesters of saturated or unsaturated C₆ -C₃₀ -monocarboxylic acids areused as alkyl carboxylates.